This invention relates to an improved automotive hydraulic pressure brake system in which brake fluid pressure is ordinarily supplied from a pump as a main pressure source, but additional fluid pressure is supplied from a master cylinder when it is necessary to increase brake pressure at a rapid pace, such as for quick brake, at low temperature causing high brake fluid viscosity and upon pump performance degradation.
To optimally control the behavior of the vehicle, vehicle brake systems are increasing which can control the braking effect electrically. For example, the brake system disclosed in Japanese patent 2590825 has, besides a master cylinder for generating fluid pressure when a brake pedal is depressed, a fluid pressure source comprising a pump and an accumulator for supplying fluid pressure to the wheel cylinders in proportion to or independently of how much the brake pedal is depressed. But since such a system needs a bulky accumulator, it is difficult to mount the entire system in an engine room.
Devices for generating brake fluid with a pump only in increasing the wheel cylinder pressure by electronic control are proposed in Japanese patent publications 9-20229 and 10-67311. With these devices, if the passage between the master cylinder and the wheel cylinders is shut off, the entire amount of brake fluid necessary for braking has to be provided by the pump, so that a pump having a large capacity is needed. Also, a delay in the pressure rise during a rapid pressure increase will be a problem, and a longer braking distance will result.
A brake system has been proposed which solves this problem. FIG. 7 is a basic circuit diagram of such a brake system. As shown, in lines connecting the master cylinder 1 to wheel cylinders 2A, 2B (letters A, B are sometimes omitted hereinbelow), an electromagnetic on-off valve 3 and a stroke simulator 4 are provided. This brake system further includes control valves (comprising pressure-increasing on-off valves 5A, 5B and pressure-reducing on-off valves 6A, 6B in this example) for controlling the wheel cylinder pressure based on command from an electronic control unit (ECU, not shown), an on-off valve 9 provided in a return line extending from the discharge port of a pump 7 (a hydraulic pump in this example) to a reservoir 8, a bypass 10 connecting the master cylinder 1 to the suction port of the pump 7, an on-off valve 11 provided in the bypass 10, and a check valve 12 provided in the return line to check a fluid flow from the bypass 10 to the reservoir 8. The on-off valves 5 and 6, which are used for antilock control, are not essential elements in this arrangement.
This brake system is also provided with fluid pressure sensors 13A, 13B, a relief valve 14 for preventing overpressure, a silencing throttle 15 and a silencer 16. Relief valve 14, throttle 15 and silencer 16 are used in this arrangement but are not essential elements.
The stroke simulator 4 has its back-pressure chamber 4b connected to the reservoir 8 to keep the chamber wet. It has a main chamber 4a. 
In the arrangement of FIG. 7, in a normal state (i.e. while the electric control unit is functioning normally), brake fluid pressure is supplied by the pump 7. When it becomes necessary to rapidly increase brake pressure and to compensate for decrease in viscosity of brake fluid due to temperature drop, fluid pressure is supplied to the wheel cylinders 2 both from the pump 7 and the master cylinder 1.
Fluid pressure can be supplied from the master cylinder by opening the on-off valve 3. But when it is repeatedly opened and closed, pulsation occurs in the brake lines. It also occurs while the pump 7 is activated. Such pulsation is transmitted to the brake pedal, thus impairing the brake feeling. If the on-off valve 3 is kept open, the brake pedal will kick back when the wheel cylinder pressure exceeds the master cylinder pressure (that is, fluid pressure produced in the master cylinder).
To solve these problems, the brake system of FIG. 7 has the bypass 10 provided with the on-off valve 11 to supply fluid pressure from the master cylinder 1 via the pump 7. But this modification posed another problem. That is, since the on-off valve 11 has to be opened in order to rapidly increase pressure, the check valve 12 is closed, so that fluid cannot be supplied to the pump 7 from the reservoir 8. Thus, a large amount of fluid has to be supplied from the master cylinder. This significantly increases the stroke of the brake pedal 17 compared with when the on-off valve 11 is closed for increase in the brake pressure at a moderate rate.
Further, while the valve 11 is open for rapid pressure increase, the pump sucks fluid discontinuously from the master cylinder. Pulsation in the brake lines is thus directly transferred to the brake pedal. Although the throttle 15 provided at the discharge side of the pump will suppress pulsation and noise, it makes it difficult to increase fluid pressure at a rapid rate.
An object of this invention is to provide a hydraulic pressure brake system that can eliminate a delay in the pressure rise during a rapid pressure increase and worsening of the pedal feeling, and in which the responsiveness is improved by increasing the suction efficiency of the pump during a rapid pressure increase or at a low temperature.
According to the invention, there is provided an automotive hydraulic pressure brake system comprising a master cylinder for producing fluid pressure corresponding to a force applied to a brake pedal, a reservoir, wheel cylinders, a power pump having an inlet port connected to the reservoir, an on-off valve provided in a fluid line connecting the master cylinder to the wheel cylinders, the on-off valve being closed to supply brake fluid pressure to the wheel cylinders from the pump while an electric control unit of the system is functioning normally, characterised in that a bypass communicating the master cylinder to the wheel brake cylinders while bypassing the on-off valve is provided, and a check valve or a relief valve for allowing only a fluid flow from the master cylinder toward the wheel brake cylinders, and a shut-off valve are provided in the bypass. (This is a first embodiment.)
Also, with the system of FIG. 7, the fluid discharged from the back-pressure chamber of the stroke simulator is returned to the reservoir without being utilized effectively.
According to the invention, there is also provided an automotive fluid pressure brake system comprising a master cylinder for producing fluid pressure corresponding to a force applied to a brake pedal, a reservoir, wheel cylinders, a power pump having an inlet port connected to the reservoir, an on-off valve provided in a fluid line connecting the master cylinder to the wheel cylinders, a stroke simulator having a main chamber communicating with a line connecting the master cylinder to the on-off valve and a back-pressure chamber communicating with the reservoir, the on-off valve being closed to supply brake fluid pressure to the wheel brake cylinders from the pump while an electric control unit of the system is functioning normally, characterized in that a throttle is provided in a circuit connecting the back-pressure chamber of the stroke simulator to the reservoir, that the system further comprises a first suction passage extending from a circuit between the throttle and the back-pressure chamber to the suction port of the pump, and a second suction passage communicating the reservoir with the first suction passage, and a check valve provided in the second suction passage to allow only a fluid flow from the reservoir toward the pump. (This is a second embodiment.)
If a bypass having a check valve or a relief valve and a shut-off valve provided in the system of the first embodiment is added to the system of the second embodiment, it will be a more preferable system (this system is a third embodiment).
If as in the system of FIG. 7, a throttle 15 is provided in the systems of the first and third embodiments on the discharge side of the pump, the bypass is preferably provided in a circuit between the throttle and the wheel cylinders.
As the stroke simulator in the system of the second or third embodiment, if one is employed which can amplify the amount of fluid flowing out of its back-pressure chamber relative to the amount of fluid flowing into its main chamber, it is possible to further increase the effect of this invention.
In the brake system of the first embodiment, if the brake pressure has to be increased rapidly but the pump alone cannot supply the brake pressure at a required speed, the ECU opens the shut-off valve in the bypass to supply master cylinder pressure through the bypass to the wheel cylinders. In addition to that, the entire fluid sucked by the pump from the reservoir is supplied to the wheel cylinders, so that delay in the pressure increase is minimized.
Since the check valve in the bypass closes when the wheel cylinder pressure exceeds the master cylinder pressure, there is no need to repeatedly open and close the shut-off valve, so that the pedal feeling will not worsen. Also, since the check valve in the bypass closes, the kick-back of the pedal is prevented while the wheel cylinder pressure is increasing. Further, since fluid replenishment from the master cylinder stops at the moment when the check valve closes, the pedal stroke is suppressed.
If a relief valve, which opens when a predetermined differential pressure is produced, is used instead of a check valve, pulsation due to activation of the pump will be less liable to be transferred. This further improves the pedal feeling.
With the system in which a throttle is provided in the discharge circuit of the pump, if make-up fluid from the master cylinder is introduced into a circuit between the throttle and the pump, the degree of improvement in the responsiveness upon sharp pressure increase will decrease due to the influence of the throttle. Thus, fluid supply from the master cylinder is preferably guided into a circuit between the throttle and the wheel cylinders.
When the brake pedal is depressed, fluid discharged from the master cylinder will flow into the main chamber of the stroke simulator. This causes brake fluid to be discharged out of the back-pressure chamber of the stroke simulator. In the system of FIG. 7, the discharged fluid flows into the reservoir without being used effectively.
In contrast, in the brake system of the second embodiment, by the action of the throttle provided between the back-pressure chamber and the reservoir, the higher the viscosity of brake fluid, and the sharper the depressing of the brake pedal and thus the larger the amount of discharged flow from the back-pressure chamber per unit time, the larger the fluid pressure in the circuit upstream of the throttle. The circuit upstream of the throttle is connected to the suction port of the pump through the first suction passage. Thus, during a rapid pressure increase or at a low temperature causing low viscosity of brake fluid, pressurized brake fluid is pushed in while the pump is working, so that the suction efficiency of the pump improves. This prevents a delay in pressure increase during sharp pressure increase or at a low temperature.
Also, in this arrangement, since a throttle is inexpensive compared with the on-off valve 11 in the system of FIG. 7, an increase in the cost is avoided. Further, while with the system of FIG. 7, fluid is supplied to the pump from the master cylinder while the on-off valve 11 is open, with the system of the second embodiment, when the pressure in the first suction passage drops due to sucking of fluid by the pump and partial flow-out of fluid via the throttle, the check valve in the second suction passage opens, so that sucking of fluid by the pump is carried out from the reservoir. Thus, the pedal stroke during sharp pressure increase will not increase extremely compared with during moderate pressure increase.
If a fluid amount amplifier is used as the stroke simulator, the amount of fluid discharged from the back-pressure chamber increases above the amount of fluid flowing into the main chamber, so that the suction efficiency of the pump further improves. This further reduces the stroke of the pedal.
The brake system of the third embodiment exhibits the functions and effects of both of the systems of the first and second embodiments, so that the pressure increase is faster.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: